qemu-sockets: improve error reporting in unix_listen_opts

Coverity complains about not checking the returned value of mkstemp.  While
at it, also improve error checking for snprintf, and refine error messages
in general.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

util/qemu-sockets.c

@@ -694,7 +694,7 @@ int unix_listen_opts(QemuOpts *opts, Error **errp)
 
     sock = qemu_socket(PF_UNIX, SOCK_STREAM, 0);
     if (sock < 0) {
-        error_setg_errno(errp, errno, "Failed to create socket");
+        error_setg_errno(errp, errno, "Failed to create Unix socket");
         return -1;
     }
 
@@ -703,9 +703,15 @@ int unix_listen_opts(QemuOpts *opts, Error **errp)
     if (path && strlen(path)) {
         snprintf(un.sun_path, sizeof(un.sun_path), "%s", path);
     } else {
-        char *tmpdir = getenv("TMPDIR");
-        snprintf(un.sun_path, sizeof(un.sun_path), "%s/qemu-socket-XXXXXX",
-                 tmpdir ? tmpdir : "/tmp");
+        const char *tmpdir = getenv("TMPDIR");
+        tmpdir = tmpdir ? tmpdir : "/tmp";
+        if (snprintf(un.sun_path, sizeof(un.sun_path), "%s/qemu-socket-XXXXXX",
+                     tmpdir) >= sizeof(un.sun_path)) {
+            error_setg_errno(errp, errno,
+                             "TMPDIR environment variable (%s) too large", tmpdir);
+            goto err;
+        }
+
         /*
          * This dummy fd usage silences the mktemp() unsecure warning.
          * Using mkstemp() doesn't make things more secure here
@@ -713,13 +719,19 @@ int unix_listen_opts(QemuOpts *opts, Error **errp)
          * to unlink first and thus re-open the race window.  The
          * worst case possible is bind() failing, i.e. a DoS attack.
          */
-        fd = mkstemp(un.sun_path); close(fd);
+        fd = mkstemp(un.sun_path);
+        if (fd < 0) {
+            error_setg_errno(errp, errno,
+                             "Failed to make a temporary socket name in %s", tmpdir);
+            goto err;
+        }
+        close(fd);
         qemu_opt_set(opts, "path", un.sun_path);
     }
 
     unlink(un.sun_path);
     if (bind(sock, (struct sockaddr*) &un, sizeof(un)) < 0) {
-        error_setg_errno(errp, errno, "Failed to bind socket");
+        error_setg_errno(errp, errno, "Failed to bind socket to %s", un.sun_path);
         goto err;
     }
     if (listen(sock, 1) < 0) {